<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .wrapper {
            width: 400px;
            height: 300px;
            border: 1px solid #000;
            font-size: 0;

        }
        .btn {
            position: absolute;
            width: 10px;
            height: 10px;
            background-color: #000;
            right: -10px;
            top: 0;
        }
    </style>
</head>
<body>
    <div class="wrapper">

        <div class="btn"></div>
    </div>
</body>
</html>